In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using an acid crystalline zeolite, such as ZSM-5 type catalyst, process conditions can be varied to favor the formation of either gasoline or distillate range products. At moderate temperature and relatively high pressure, the conversion conditions favor distillate range product having a normal boiling point of at least 165.degree. C. (330.degree. F.). Lower olefinic feedstocks containing C.sub.2 -C.sub.6 alkenes may be converted selectively; however, the distillate mode conditions do not convert a major fraction of ethylene. While propene, butene-1 and others may be converted to the extent of 50 to 95% in the distillate mode, only about 10 to 20% of the ethylene component will be consumed.
In the gasoline mode, ethylene and the other lower olefins are catalytically oligomerized at higher temperature and moderate pressure. However, coking of the catalyst is accelerated by the higher temperature. Under these conditions ethylene conversion rate is greatly increased and lower olefin oligomerization is nearly complete to produce an olefinic gasoline comprising hexene, heptene, octene and other C.sub.6.sup.+ hydrocarbons in good yield. To avoid excessive temperatures in the exothermic reactors, the lower olefinic feed may be diluted. In the distillate mode operation, olefinic gasoline may be recycled and further oligomerized, as disclosed in U.S. Pat. No. 4,211,640 (Garwood and Lee). In eiter mode, the diluent may contain light hydrocarbons, such as C.sub.3 -C.sub.4 alkanes, present in the feedstock and/or recycled from the debutanized product.
In U.S. Pat. Application Ser. No. 481,705, filed Apr. 4, 1983, now U.S. Pat. No. 4,433,185 and incorporated herein by reference, a two stage catalytic process is disclosed for converting lower olefins at elevated temperature and pressure, with unconverted reactant, mainly ethylene, from a first stage being completely converted at higher temperature in a second stage. Although, the same type catalyst (H-ZSM-5) is employed in each stage, significant differences in the operating temperatures and catalyst use contribute to different rates of inactivation, largely due to coking.
The present invention takes advantage of the accelerated aging rate for hydrocarbon conversion catalysts operating under process conditions which produce coke deposits. Increased coking will decrease conversion at a given temperature, and it is conventional practice to increase process temperature to maintain the desired level of conversion. In the two stage olefin oligomerization process contemplated in the preferred embodiment herein, the primary stage feedstock is selectively converted over highly active ZSM-5 type catalyst at moderate temperature and high pressure. Under these conditions C.sub.3.sup.+ olefin primary reactants are converted efficiently in major amount to a highly desirable distillate product; however, only a minor amount of ethylene is converted at primary stage temperature.
By recovering unreacted ethylene and other light olefins from the primary stage, a second reactant stream from high temperature conversion can utilize coked catalyst that would no longer be suitable for lower temperature use due to loss of activity.
In order to maintain the MOGD plant in continuous operation, it is necessary to either replace or regenerate spent catalyst periodically. Advantageously, a single reactor can serve the entire multi-stage complex by appropriate sequencing of a plurality of fixed bed reactors. By employing the same type of catalyst bed in similar amount and configuration for each reactor, the same reactor shell can be switched to serve in any of the process positions according to need.